Equipment for Engineering Education

Transcription

1 Equipment for Engineering Education Operating Instructions HM70.24 Boundary Layer Plate G.U.N.T. Gerätebau GmbH P.O. Box 25 D-2288 Barsbüttel Germany Phone (040) Fax (040)

2 Table of Contents Description Experiments Installation of model Plate replacement Performance of experiment Evaluation Technical data /96

3 Description The model HM70.24 is used for investigating the boundary layer on a plate subjected to longitudinal flow. The unit consists of two optionally insertable plates () of differing roughness which can be fitted in parallel with the flow in the working section (2) of the wind tunnel HM70 and a Pitot tube probe () for measuring velocity. The plates () can be moved in axial direction by way of a gear drive (4) for measuring velocity after different flow lengths. For exact measurement of the flow field in vertical direction, the Pitot tube probe () can be adjusted with extreme precision via a micrometer spindle (5). A second measurement connection (6) for detecting the static pressure is provided at the bracket for the Pitot tube probe. 4

4 2 Experiments In the case of flow around a plate, both laminar and turbulent boundary layers are encountered. At first the boundary layer is laminar with a virtually linear velocity profile resulting from the viscosity of the fluid. The laminar boundary layer may become turbulent after a certain flow distance. A measure of this is the Reynolds number Re formed by the distance x u of the transition point from the front edge of the plate. Re crit. = w x u. ν Re crit. assumes values between 0 5 and 0 6. y w w Laminar Transition zone Turbulent Boundary layer at the plate As the exchange of energy between zones close to the wall and zones away from the wall is now more intensive in the boundary layer on account of the turbulence, the turbulent boundary layer is thicker than the laminar one and the velocity profile versus height y is no longer linear but rather convex. The boundary layer thickness at a given location decreases with velocity. Furthermore, the thick- 2

5 ness increases with increasing distance x from the front edge. p dyn. p stat. p tot. For measurement, velocity is determined by way of the dynamic pressure p dyn., which represents the difference between the total pressure/stagnation pressure p tot. at the Pitot tube probe and the static pressure p stat., measured at the measurement connection on the bracket of the Pitot tube probe. The inclined-tube manometer forms the difference and thus gives a direct indication of the dynamic pressure. 2. Installation of model View from below The velocity is calculated as follows w = 2 p dyn ρ. Installing plate in working section - Insert plate () with bracket in working section such that plate faces upwards and its bevelled edge forwards. - Use the 4 bolts (2) provided to secure the bracket through the dia. 5. mm holes. - Insert rotary knob () for plate adjustment in centre hole Inserting micrometer spindle with Pitot tube probe - Loosen knurled screw (4) at bracket of Pitot tube probe (5). Pull probe upwards such that it does not protrude more than 00 mm into the working section. This will protect the sensitive probe from hard contact to the plate (). - Insert micrometer spindle (6) with Pitot tube probe into working section from above and fasten from inside with the knurled screw (7) provided.

6 ATTENTION: The Pitot tube is highly sensitive and is not to be bent or kinked. - Screw in probe bracket with micrometer adjusting screw (6) to roughly mm before lower stop. Insert probe (5) until it just makes contact with the plate (), align in flow direction and secure again at probe bracket with knurled screw (4). - Connect inclined-tube manometer as shown. 2.2 Plate replacement 2 ATTENTION: Before running up wind tunnel, make sure all components are securely fastened and that there are no loose parts in the working section. Replacement of the plate with one of differing roughness involves removing the model from the working section and setting it down with the underside facing upwards. - Remove front securing bolts () of racks (2) on either side (use dia. 2 mm pin to loosen bolts) and push out plate to rear. Attention: Brake springs () may drop out on removing the plate. - Likewise remove front securing bolts (4) (bevelled side) of racks on new plate and press plate into bracket from rear. Make sure that bevelled side (5) is facing forwards and upwards

7 ATTENTION: Make sure brake springs () are properly seated in their mounts. - After inserting plate, fit and tighten securing bolts (4) of racks. 2. Performance of experiment p dyn ATTENTION: Before running up wind tunnel, make sure all components are securely fastened and that there are no loose parts in the working section. - Measure ambient temperature, atmospheric pressure and relative humidity and thus establish current air density. - Set inclined-tube manometer to zero by shifting scale. - Run up wind tunnel to desired speed. - Use adjusting knob to move plate to rear position so that Pitot tube probe at front edge is at x=0 mm. - Lower probe onto surface of plate and record first measured value for p dyn. On account of the probe diameter of d=0.7 mm, this value corresponds to a height of y=0.5 mm. - Record a measured value every 0.25 mm up to a height of y=.5 mm. One measured value every mm suffices up to a height of y=0 mm. - Repeat procedure for other positions (e.g. x= 50, 00, 50mm). 5

8 2.4 Evaluation The table below lists the results of a specimen experiment with a smooth plate (roughness R z = 25µm). Velocity distribution in the boundary layer, smooth plate, roughness Rz = 25µm w = 2.6m/s, density ρ =.2kg/m Distance from the plate y in mm x= 0 mm x = 50 mm x = 00 mm x = 50 mm Pressure p dyn. in Pa Velocity w in m/s Pressure p dyn. in Pa Velocity w in m/s Pressure p dyn. in Pa Velocity w in m/s Pressure p dyn. in Pa Velocity w in m/s The velocity distributions w (x,y) are represented as a graph in the following. The distance y, at which 90% of the undisturbed velocity w is attained, was taken as a measure of the boundary layer thickness. 6

9 w Distance y from surface in mm w (x,y) Distand x from front edge in mm Velocity distribution and boundary layer thickness at a plane plate Boundary layer thickness 7

10 Technical data Bracket Width: 290 mm Length: 220 mm Attachment holes geared to working section of wind tunnel HM70 Plate Length : 250 mm Thickness : mm Horizontal adjustment: 80 mm Surface roughness Rz smooth 25 µm rough 400 µm Pitot tube probe with micrometer spindle: Probe diameter: dia. 0.7 mm Vertical adjustment: 25 mm Resolution: 0.0 mm Hose connections: dia. 2 mm Attachment holes geared to working section of wind tunnel HM70 8