Orientation on quantitative IR-thermografy in wall-shear stress measurements

Orientation on quantitative IR-thermografy in wall-shear stress measurements

Mayer, R.

Aerospace Engineering

1998-12-31

Wall-shear stresses are highly important in the aerodynamic design of aircraft, because they determine the drag and thus the fuel consumption of an airplane. Due to this importance many different measurement techniques have been developed. Most of these techniques are intrusive, which means that the flow is disturbed by the presence of a measurement probe. The hot film technique is non-intrusive, because hot films measure the heat transfer from an electrically heated surface of an object to the flow, which is related to the wall-shear stress. Using the theory of the hot film technique, we have developed a new non-intrusive wall-shear stress measurement technique, which is based on quantitative IR-thermography. In this technique a hot spot is externally generated by a laser. The surface temperature measurements, from which the heat flux to the flow is derived, is measured externally by an IR-camera. The external heating and the extern al temperature measurement provides three main advantages: * the measurement is non-intrusive * the measurement point can easily be varied * th is technique can be applied in flight tests This measurements technique has been tested for a laminar flow along a flat plate in a wind-tunnel. The laser generates a hot spot on the plate until the steady state condition is reached. After turning off the laser the IR-camera monitors the temperature decay. From these data the heat flux to the flow and the wall-shear stress can be derived. The results show that it is indeed possible to apply quantitative IR-thermography to measure local wall-shear stresses. The obtained accuracy of the measurement technique is +/- 10% for free stream velocities larger than 10 m/s. However it has to be noted that this technique has some limitations due to the capabilities of the IR-camera, such as the spatial resolution and the signal to noise ratio. In the near future this measurement technique will be extended to flows with nonzero pressure gradients and with turbulence.

gas flow measurement
gas boundary-layer flow
aircraft dynamics
wind tunnels

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Delft University Press

9040715726

Series 01: Aerodynamics, 09

Institutional Repository

book

(c) 1998 Faculty of Aerospace Engineering