During incident flow of bodies fluids such as air “stick” to the surface of the body and form the so-called boundary layer. The kind of flow within the boundary layer – laminar or turbulent – significantly affects the drag. The findings from studying the boundary layer are taken into consideration when designing aerofoils (aircraft construction), turbine blades (turbine construction) and hull, rudder and propeller blades (shipbuilding).

The HM 170.24 experimental unit – used in the wind tunnel HM 170 – allows the boundary layer on a flat plate to be studied. For this purpose, the plate is inserted into the measuring section of the wind tunnel. The air flows along the plate, parallel to the surface. Two plates with different surface roughnesses are available to demonstrate the effect of surface conditions on the boundary layer.

A vertically movable Pitot tube, adjusted using a micrometer screw, measures the total pressures at various distances from the plate surface. The plate can be moved horizontally, to enable the recording of total pressures along the flow. An additional measuring point measures the static pressure. Both measuring points are connected to a manometer. The velocity can then be calculated from the displayed dynamic pressure. To indicate the pressure, the following units are optionally available: inclined tube manometer included in HM 170, differential pressure manometer HM 170.53, electronic pressure measurement HM 170.55 or system for data acquisition HM 170.60.

The measured values for the dynamic pressure depending on the distance of the Pitot tube to plate front edge can be displayed graphically by using the system for data acquisition HM 170.60 and the electronic displacement measurement HM 170.61.