In order to calculate the efficiency of a hydro power plant some quantities needs to be measured. One of them is the flow rate, which can be difficult to measure with good accuracy. There are several methods for that task. Gibson's method is one of them and has the advantage to be economical and easily installed at the site. In order to achieve accurate results with this method, some criterions must be fulfilled. Two of these criterions are the measuring length must be larger than 10 m and the mean initial velocity times the measuring length should be larger than 50 m2/s. These criterions are rarely fulfilled in low head hydro power plants.This paper presents a numerical analysis of the effects of the connecting tubing between the pressure taps at the conduit and the differential pressure sensor. A one dimensional code programmed in Matlab is used for the numerical simulations where the governing equations, continuity and equation of motion, are discretizised with a second order Godunov-type scheme. The simulations are made for different initial flow rates and different valve closures. The result shows that the tubing has an overall small influence on the calculated flow rate error. A positive bias error appears in all tested cases and has a maximum (1.2 to 1.3 % depending on the test case) at the smallest Reynolds number.

The density distribution in the air surrounding an airgun muzzle is visualized using pulsed TV holography. A ruby laser emitting short coherent light pulses (30 ns) is used as light source. Pulsed holograms are captured by a CCD-camera and their optical phase difference is evaluated by means of the Fourier transform method. A number of experiments are performed where the event is recorded at different instants of time as the lead bullet is about to leave the muzzle. Phase maps showing the integrated density distribution are presented. A jet with decreasing density ahead of the bullet can be observed. At the moment the bullet leaves the muzzle, a spherical sound pressure wave starts to propagate out into the air. The density in the jet in front of the traveling bullet can be calculated by means of full shock-wave theory. The theory confirms the decrease in density observed in the measurements