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Saemi, S., Sundström, J., Cervantes, M. & Raisee, M. (2019). Evaluation of transient effects in the pressure-time method. Flow Measurement and Instrumentation, 68, Article ID 101581.
Open this publication in new window or tab >>Evaluation of transient effects in the pressure-time method
2019 (English)In: Flow Measurement and Instrumentation, ISSN 0955-5986, E-ISSN 1873-6998, Vol. 68, article id 101581Article in journal (Refereed) Published
##### Abstract [en]

The pressure-time is a method for measuring the flow rate in closed conduits and is typically used in hydropower applications. The scope of the present paper is to examine the flow physics in the pressure-time method using experimental measurements and two-dimensional numerical simulations. The Unsteady Reynolds-averaged Navier–Stokes (URANS) equations and the low-Re k-ω SST turbulence model are employed for the simulations. The contributions of inertia, pressure gradient, viscous and turbulent shear stresses are investigated in the flow during a pressure-time measurement. It is shown that away from the wall and at the first times, the turbulent shear stress balances with the pressure gradient. By increasing the time, the inertia effect becomes dominant and balances with the pressure gradient and turbulent shear stress. Close to the wall, both viscous and turbulent shear stresses are the dominant terms which are decreasing by increasing the time. It is also shown that the prediction of the friction losses can be improved by modeling the dependence of the friction factor on the dimensionless parameter instead of the Reynolds number.

Elsevier, 2019
##### Keywords
Pressure-time method, Flow rate measurement, Transient flow, CFD, Experiments
##### National Category
Fluid Mechanics and Acoustics
Fluid Mechanics
##### Identifiers
urn:nbn:se:ltu:diva-71997 (URN)10.1016/j.flowmeasinst.2019.101581 (DOI)000483649300016 ()
##### Note

Available from: 2018-12-11 Created: 2018-12-11 Last updated: 2019-09-24Bibliographically approved
Sundström, J., Saemi, S., Raisee, M. & Cervantes, M. (2019). Improved frictional modeling for the pressure-time method. Flow Measurement and Instrumentation, 69, Article ID 101604.
Open this publication in new window or tab >>Improved frictional modeling for the pressure-time method
2019 (English)In: Flow Measurement and Instrumentation, ISSN 0955-5986, E-ISSN 1873-6998, Vol. 69, article id 101604Article in journal (Refereed) Published
##### Abstract [en]

The pressure-time method is classified as a primary method for measuring discharge in hydraulic machinery. The uncertainty in the discharge determined using the pressure-time method is typically around ±1.5 %; however, despite dating back almost one hundred years in time, there still exists potential to reduce this uncertainty. In this paper, an improvement of the pressure-time method is suggested by implementing a novel formulation to model the frictional losses arising in the evaluation procedure. By analyzing previously obtained data from CFD, laboratory and full-scale pressure-time measurements it is shown that the new friction model improves the accuracy of the flow rate calculation by approximately 0.1–0.2% points, compared to currently utilized friction models. Despite being a small absolute improvement, the new friction model presents an important development of the pressure-time method because the relative improvement is significant.

Elsevier, 2019
##### Keywords
Pressure-time method, Transient friction modeling, Pipe flow
##### National Category
Fluid Mechanics and Acoustics
Fluid Mechanics
##### Identifiers
urn:nbn:se:ltu:diva-75599 (URN)10.1016/j.flowmeasinst.2019.101604 (DOI)000496341600005 ()2-s2.0-85070506497 (Scopus ID)
##### Note

Available from: 2019-08-20 Created: 2019-08-20 Last updated: 2020-04-22Bibliographically approved
Sundström, L. J. & Cervantes, M. (2018). Characteristics of the wall shear stress in pulsating wall-bounded turbulent flows. Experimental Thermal and Fluid Science, 96, 257-265
Open this publication in new window or tab >>Characteristics of the wall shear stress in pulsating wall-bounded turbulent flows
2018 (English)In: Experimental Thermal and Fluid Science, ISSN 0894-1777, E-ISSN 1879-2286, Vol. 96, p. 257-265Article in journal (Refereed) Published
##### Abstract [en]

A pulsating turbulent pipe flow has been investigated experimentally using hot-film anemometry and particle image velocimetry. Particularly, a `paradoxical phenomenon' that is known to occur for a range of forcing frequencies and time-averaged Reynolds numbers has been investigated in detail. The paradoxical phenomenon is that the oscillating component of the wall shear stress exhibits a smaller amplitude in a turbulent flow compared to in a laminar flow exposed to the same oscillation in the pressure gradient. In here, the phenomenon is explained by splitting the response of the wall shear stress into one contribution resulting from the imposed pressure gradient $\widetilde{\tau}_p$, and a second contribution resulting from the oscillating Reynolds shear stress,$\widetilde{\tau}_t$ At the conditions of maximum reduction of the wall shear stress amplitude, $\widetilde{\tau}_{t}$ and $\widetilde{\tau}_{p}$ are nearly 136 degrees out of phase. The contributions are thus interfering destructively, this being the ultimate reason for the reduced amplitude. It is also shown that the level of reduction is dependent on the imposed forcing amplitude, this in turn residing from a dependence of the time-development of the oscillating Reynolds shear stress on the forcing amplitude.

Elsevier, 2018
##### National Category
Fluid Mechanics and Acoustics
Fluid Mechanics
##### Identifiers
urn:nbn:se:ltu:diva-67866 (URN)10.1016/j.expthermflusci.2018.02.036 (DOI)000432235300025 ()2-s2.0-85044007965 (Scopus ID)
##### Note

Available from: 2018-03-06 Created: 2018-03-06 Last updated: 2018-06-11Bibliographically approved
Sundström, J. & Cervantes, M. (2018). Laminar similarities between accelerating and decelerating turbulent flows. International Journal of Heat and Fluid Flow, 71, 13-26
Open this publication in new window or tab >>Laminar similarities between accelerating and decelerating turbulent flows
2018 (English)In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 71, p. 13-26Article in journal (Refereed) Published
##### Abstract [en]

An experimental study of a pipe flow ramping monotonically between two turbulent states has been undertaken. Ensemble-averaged mean and turbulent flow quantities obtained from two-component particle image velocimetry and hot-film anemometry measurements have been presented. It is shown that the initial developments of the mean and turbulence quantities in linearly as well as impulsively accelerating and decelerating flows are similar. Specifically, the mean perturbation velocity (defined as the surplus/deficit from the initial value) can be described using self-similar expressions. The duration of this initial stage, when normalized appropriately, is shown to be approximately invariant of the type of transient imposed on the bulk flow. Data from studies of linearly accelerating and decelerating flows as well as impulsively accelerating and decelerating flows have been used to validate the results, covering four orders of magnitude of the dimensionless parameter $\delta$. The highest initial Reynolds number (31,000) is, however, relatively low thus requiring further studies at high Reynolds numbers to assure the universality of the results. We have also shown that the time-development of the mean and turbulent quantities between an accelerating and a decelerating flow looses their similarity as the transient proceeds beyond the initial stage. The departure was explained by the time-evolvement of the production of turbulence kinetic energy, which exhibit differences between the two types of transients.

Elsevier, 2018
##### National Category
Fluid Mechanics and Acoustics
Fluid Mechanics
##### Identifiers
urn:nbn:se:ltu:diva-67867 (URN)10.1016/j.ijheatfluidflow.2018.03.005 (DOI)000435428900002 ()2-s2.0-85044133898 (Scopus ID)
##### Note

Available from: 2018-03-06 Created: 2018-03-06 Last updated: 2018-07-26Bibliographically approved
Sundström, J. & Cervantes, M. J. (2018). On the Similarity of Pulsating and Accelerating Turbulent Pipe Flows. Flow Turbulence and Combustion, 100(2), 417-436
Open this publication in new window or tab >>On the Similarity of Pulsating and Accelerating Turbulent Pipe Flows
2018 (English)In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 100, no 2, p. 417-436Article in journal (Refereed) Published
##### Abstract [en]

The near-wall region of an unsteady turbulent pipe flow has been investigated experimentally using hot-film anemometry and two-component particle image velocimetry. The imposed unsteadiness has been pulsating, i.e., when a non-zero mean turbulent flow is perturbed by sinusoidal oscillations, and near-uniformly accelerating in which the mean flow ramped monotonically between two turbulent states. Previous studies of accelerating flows have shown that the time evolution between the two turbulent states occurs in three stages. The first stage is associated with a minimal response of the Reynolds shear stress and the ensemble-averaged mean flow evolves essentially akin to a laminar flow undergoing the same change in flow rate. During the second stage, the turbulence responds rapidly to the new flow conditions set by the acceleration and the laminar-like behavior rapidly disappears. During the final stage, the flow adapts to the conditions set by the final Reynolds number. In here, it is shown that the time-development of the ensemble-averaged wall shear stress and turbulence during the accelerating phase of a pulsating flow bears marked similarity to the first two stages of time-development exhibited by a near-uniformly accelerating flow. The stage-like time-development is observed even for a very low forcing frequency; &#x03C9;+=&#x03C9;&#x03BD;/u&#x00AF;&#x03C4;2=0.00073" role="presentation">ω+=ων/u¯¯¯2τ=0.00073 (or equivalently, ls+=2/&#x03C9;+=52" role="presentation">l+s=2/ω+−−−−√=52), at an amplitude of pulsation of 0.5. Some previous studies have considered the flow to be quasi-steady at ls+=52" role="presentation">l+s=52; however, the forcing amplitude has been smaller in those studies. The importance of the forcing amplitude is reinforced by the time-development of the ensemble-averaged turbulence field. For, the near-wall response of the Reynolds stresses showed a dependence on the amplitude of pulsation. Thus, it appears to exist a need to seek alternative similarity parameters, taking the amplitude of pulsation into account, if the response of different flow quantities in a pulsating flow are to be classified correctly.

Springer, 2018
##### National Category
Fluid Mechanics and Acoustics
Fluid Mechanics
##### Identifiers
urn:nbn:se:ltu:diva-65766 (URN)10.1007/s10494-017-9855-5 (DOI)000424665400005 ()30069140 (PubMedID)
##### Note

Available from: 2017-09-22 Created: 2017-09-22 Last updated: 2018-08-06Bibliographically approved
Sundström, J. & Cervantes, M. (2017). The response of the wall shear stress in uniformly and nonuniformly accelerating pipe flows. In: : . Paper presented at 10th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2017, Chicago, United States, ; 6-9 July 2017. International Symposium on Turbulence and Shear Flow Phenomena, 1
Open this publication in new window or tab >>The response of the wall shear stress in uniformly and nonuniformly accelerating pipe flows
2017 (English)Conference paper, Published paper (Refereed)
##### Abstract [en]

Wall shear stress measurements in an accelerating turbulent pipe flow have been performed. Four different imposed accelerations have been studied including one uniform and three nonuniform ones. The initial-to-final Reynolds numbers as well as the acceleration time were approximately the same for each case, thereby isolating the effect of the type of acceleration. Data have been taken using hot-film anemometry, and it has been established that the time-development for each case is qualitatively similar, although there are significant quantitative differences between each case. The previously established view that the time-development of an accelerating flow resemble a laminar-to-turbulent bypass transition is confirmed. An explanation for the transitional behavior is sought through the Poisson equation describing the pressure fluctuations. It is postulated that the fast pressure induces asymmetries in the time-development of the wall-normal velocity fluctuations thereby leading to a route to transition. Due to lack of data, however, the proposed explanation cannot be confirmed or rejected, for that, further experimental as well as numerical studies have to be performed

##### Place, publisher, year, edition, pages
International Symposium on Turbulence and Shear Flow Phenomena, 2017
##### National Category
Fluid Mechanics and Acoustics
Fluid Mechanics
##### Identifiers
urn:nbn:se:ltu:diva-66627 (URN)2-s2.0-85033222803 (Scopus ID)9780000000002 (ISBN)
##### Conference
10th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2017, Chicago, United States, ; 6-9 July 2017
Available from: 2017-11-17 Created: 2017-11-17 Last updated: 2017-11-24Bibliographically approved
Sundström, J. & Cervantes, M. (2017). Transient wall shear stress measurements and estimates at high Reynolds numbers. Flow Measurement and Instrumentation, 58, 112-119
Open this publication in new window or tab >>Transient wall shear stress measurements and estimates at high Reynolds numbers
2017 (English)In: Flow Measurement and Instrumentation, ISSN 0955-5986, E-ISSN 1873-6998, Vol. 58, p. 112-119Article in journal (Refereed) Published
##### Abstract [en]

Transient wall shear stress measurements using hot-film anemometry have been performed in a large-scale laboratory setup at high Reynolds numbers. Starting from Reynolds numbers 1.7×1061.7×106 and 0.7×1060.7×106, the flow was brought to a complete rest by closing a knife gate thus replicating a pressure-time (also know as Gibson) flow rate measurement in a hydropower plant. Ensemble-averaged mean wall shear stresses obtained from 22 repeated runs have been compared with estimates obtained using the pressure-time method. The objective of the work has been to assess the accuracy of the frictional formulation entering the pressure-time integral. It is shown that both the standard method, a quasi-steady approach as well as the recently introduced unsteady method all reproduce the measured wall shear stresses quantitatively during most of the transient. The last phase, following the complete closure of the gate, which is characterized by a slow decay towards zero shear stress at the wall is, however, not captured by the available methods. In general, the unsteady formulation produces the smallest flow rate estimation error, which in turn, implies the best modeling of the frictional losses.

Elsevier, 2017
##### National Category
Fluid Mechanics and Acoustics
Fluid Mechanics
##### Identifiers
urn:nbn:se:ltu:diva-66149 (URN)10.1016/j.flowmeasinst.2017.10.003 (DOI)000419419600014 ()2-s2.0-85032350533 (Scopus ID)
##### Note

Available from: 2017-10-16 Created: 2017-10-16 Last updated: 2018-03-06Bibliographically approved
Sundström, J., Mulu, B. & Cervantes, M. (2016). Wall friction and velocity measurements in a double-frequency pulsating turbulent flow (ed.). Paper presented at . Journal of Fluid Mechanics, 788, 521-548
Open this publication in new window or tab >>Wall friction and velocity measurements in a double-frequency pulsating turbulent flow
2016 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 788, p. 521-548Article in journal (Refereed) Published
##### Abstract [en]

Wall shear stress measurements employing a hot-film sensor along with laser Doppler velocimetry measurements of the axial and tangential velocity and turbulence profiles in a pulsating turbulent pipe flow are presented. Time-mean and phase-averaged results are derived from measurements performed at pulsation frequencies ω+ = ων/u¯τ 2 over the range of 0.003-0.03, covering the low-frequency, intermediate and quasi-laminar regimes. In addition to the base case of a single pulsation imposed on the mean flow, the study also investigates the flow response when two pulsations are superimposed simultaneously. The measurements from the base case show that, when the pulsation belongs to the quasi-laminar regime, the oscillating flow tends towards a laminar state in which the velocity approaches the purely viscous Stokes solution with a low level of turbulence. For ω+ < 0.006, the oscillating flow is turbulent and exhibits a region with a logarithmic velocity distribution and a collapse of the turbulence intensities, similar to the time-averaged counterparts. In the low-frequency regime, the oscillating wall shear stress is shown to be directly proportional to the Stokes length normalized in wall units ls + (=√2/ω+), as predicted by quasi-steady theory. The base case measurements are used as a reference when evaluating the data from the double-frequency case and the oscillating quantities are shown to be close to superpositions from the base case. The previously established view that the time-averaged quantities are unaffected by the imposition of small-amplitude pulsed unsteadiness is shown to hold also when two pulsations are superposed on the mean flow

##### National Category
Fluid Mechanics and Acoustics
Fluid Mechanics
##### Identifiers
urn:nbn:se:ltu:diva-3505 (URN)10.1017/jfm.2015.722 (DOI)000368413600014 ()2-s2.0-84953775346 (Scopus ID)155fb13a-51af-4173-9c93-79be3ef6f44f (Local ID)155fb13a-51af-4173-9c93-79be3ef6f44f (Archive number)155fb13a-51af-4173-9c93-79be3ef6f44f (OAI)
##### Note
Validerad; 2016; Nivå 2; 20160119 (andbra)Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2018-07-10Bibliographically approved
Sundström, J., Amiri, K., Bergan, C., Cervantes, M. & Dahlhaug, O. G. (2014). LDA measurements in the Francis-99 draft tube cone (ed.). In: (Ed.), (Ed.), 27th IAHR Symposium on Hydraulic Machinery and Systems, IAHR:: Montreal, Canada, 22-26 September 2014. Paper presented at IAHR Symposium on Hydraulic Machinery and Systems : 22/09/2014 - 26/09/2014. : IOP Publishing Ltd, Article ID 22012.
Open this publication in new window or tab >>LDA measurements in the Francis-99 draft tube cone
2014 (English)In: 27th IAHR Symposium on Hydraulic Machinery and Systems, IAHR:: Montreal, Canada, 22-26 September 2014, IOP Publishing Ltd , 2014, article id 22012Conference paper, Published paper (Refereed)
##### Abstract [en]

Velocity measurements were performed in the draft tube cone of a 1:5.1 scaled model of the Tokke hydropower plant, Norway; also known as the Francis-99 model. Results from the laser Doppler anemometry measurements undertaken at three operating points will be used as validation data for an upcoming workshop on the state of the art of Francis turbine numerical simulation. With the turbine operating at the best efficiency point, a sensitivity analysis of the flow parameters head, flow rate and runner rotational speed shows that the effects on the dimensionless velocity profiles are small as long as nED and QED are held constant. The results indicate a well-functioning turbine at the best efficiency point and high load. At the part load operating point, a vortex breakdown occurs which distorts the velocity profiles and significantly lowers the turbine’s hydraulic efficiency. Frequency spectrums of each LDA signal at part load reveals a peak which is asynchronous to that of the runner angular speed. The peaks might be related to the precession of a rotating vortex rope but the characteristics of the LDA signals are different compared to previous studies involving rotating vortex ropes.

##### Place, publisher, year, edition, pages
IOP Publishing Ltd, 2014
##### Series
I O P Conference Series: Earth and Environmental Science, ISSN 1755-1307 ; 22
##### National Category
Fluid Mechanics and Acoustics
Fluid Mechanics
##### Identifiers
urn:nbn:se:ltu:diva-38235 (URN)10.1088/1755-1315/22/2/022012 (DOI)000347441900046 ()2-s2.0-84919686180 (Scopus ID)c8e9aa1e-a01f-426f-ae10-72799f2668fc (Local ID)c8e9aa1e-a01f-426f-ae10-72799f2668fc (Archive number)c8e9aa1e-a01f-426f-ae10-72799f2668fc (OAI)
##### Conference
IAHR Symposium on Hydraulic Machinery and Systems : 22/09/2014 - 26/09/2014
##### Note
Validerad; 2015; Nivå 1; 20141002 (sunjoe)Available from: 2016-10-03 Created: 2016-10-03 Last updated: 2018-07-10Bibliographically approved
Sundström, J., Mulu, B. & Cervantes, M. (2014). Wall friction and velocity measurements in multiple frequencies pulsating flow (ed.). Paper presented at International Conference on Experimental Fluid Mechanics : 12/08/2014 - 15/08/2014. Paper presented at International Conference on Experimental Fluid Mechanics : 12/08/2014 - 15/08/2014.
Open this publication in new window or tab >>Wall friction and velocity measurements in multiple frequencies pulsating flow
2014 (English)Conference paper, Oral presentation only (Refereed)
##### Abstract [en]

Turbulent pulsating flow in a 100 mm diameter pipe has been studied experimentally at a Reynolds number of 14,500. The work covers four different flow conditions; steady, pulsating flow at oscillation frequencies 0.08 and 0.4 Hz, and pulsating flow with simultaneous imposition of both frequencies. The amplitudes of the pulsations were 10 and 7.5% of the bulk flow at 0.08 and 0.4 Hz, respectively. Laser Doppler anemometry, hot-film and pressure measurements show that the mean values of velocity, wall shear stress and pressure gradient are unaffected by the imposed pulsations. In agreement with previous studies of pulsating flow, the phase averaged pressure gradient leads both the velocity and wall shear stress when a single pulsation is imposed on the mean flow. These phase leads remain virtually unchanged when the two frequencies are imposed simultaneously. The amplitude responses of the velocity, wall shear stress and pressure gradient in the combined pulsating flow is shown to be superpositions of the amplitudes from the cases of separate pulsations. There are no signs of non-linear interactions between the harmonics.

##### National Category
Fluid Mechanics and Acoustics
Fluid Mechanics
##### Identifiers
urn:nbn:se:ltu:diva-30512 (URN)45b6e094-2dfa-4cbc-88d5-5b28aaaa677b (Local ID)45b6e094-2dfa-4cbc-88d5-5b28aaaa677b (Archive number)45b6e094-2dfa-4cbc-88d5-5b28aaaa677b (OAI)
##### Conference
International Conference on Experimental Fluid Mechanics : 12/08/2014 - 15/08/2014
##### Note
Godkänd; 2014; 20140912 (sunjoe)Available from: 2016-09-30 Created: 2016-09-30 Last updated: 2017-11-25Bibliographically approved
##### Identifiers
ORCID iD: orcid.org/0000-0002-3349-601x

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