Head loss in a gate plug

# Head loss in a gate plug

The following is taken from Olsen and Oldervik (1995) "Three-dimensional numerical modeling of water flow through a gate plug", IAHR 26th. Biennial Congress, London

Today there exist several numerical models that are able to calculate the water flow field in a general three-dimensional geometry. This study investigates the performance of some of these models applied for a gate plug. The gate plug case is chosen as an example from hydraulic engineering. Physical model tests for this case exist which is used for validation of the numerical models.

The gate plug is used in hydropower tunnels as a mean of supporting a gate. The gate plug decreases the cross-sectional area of the tunnel, and consists of a contraction and an expansion region. The gate is located between these regions. Two plugs are investigated. Plug A has a relatively long expansion region, similar to a diffusor. For Plug B this expansion region is cut after 1/3 of the length. In the simulated cases the plug is located in a rectangular tunnel, 7.4 m high and 5.76 m wide. Only one quarter of the geometry is simulated, and symmetry conditions are used on two planes. The length of the geometry is 37 meters. The inlet velocity is 1.5 m/s.

The energy loss seems reasonable for case A. For case B there is more deviation between the calculated and observed energy loss. This can be due to a relatively coarse grid at the cut of the expansion. The question of number of grid cells is for many cases dependent on the speed of the numerical model and the computers available. To check this, a new grid with 8 times as many cells were calculated for case B using SSIIM. This gave a head loss of 2496 Pascal, a reduction in error from 73 % to 39 %.

For a geometry where the grid can be made fairly smooth, the numerical models give a result with an accuracy of 10-20 %. For a more complex geometry with large recirculation zones and a coarse grid the deviation between calculated and measured head loss can be over 100 %.

Case, A; Longitudinal profile of the gate plug, where the colors show the magnitude of the velocity.

Case A; Longitudinal profile of the gate plug, where the colors show the magnitude of the pressure.

Case B; Longitudinal profile of the gate plug, where the colors show the magnitude of the velocity. The number of grid cells has been increased by a factor 8.