A method for global measurement of surface friction stress of a wind tunnel model

Abstract

The application discloses a kind of wind tunnel model surface friction stress global measurement method, the purpose is to friction stress global measurement, using coating heating mode to heat model surface, after reaching thermal equilibrium, the temperature distribution of model surface is obtained using high-precision infrared thermal imager, at specified position, using mature friction stress local measurement technology to obtain local friction stress, based on the friction stress and temperature distribution along streamline, the friction stress on the whole streamline is calculated;Using mature local friction stress measurement method and high-precision infrared thermal imaging technology to measure global friction stress, the measuring equipment is relatively simple, the model surface is heated by heating coating with constant power, the relationship between friction stress size and local temperature is established, the calculation method is relatively simple, the resolution of friction stress is higher, and the resolution of infrared image is equivalent, other flow field information can be obtained simultaneously.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of wind tunnel test, in particular to a method for measuring global surface friction stress of a wind tunnel model. BACKGROUND

[0002] Wind tunnel experiment is an experimental method for installing a model of an aircraft or other object in a wind tunnel to study the gas flow and its interaction with the model, so as to understand the aerodynamic characteristics of the actual aircraft or other object. Resistance is an important evaluation index of the aerodynamic performance of the aircraft, and is directly related to the range and fuel consumption rate of the aircraft. Resistance mainly includes pressure difference resistance and friction resistance. Among them, the friction resistance can account for 30% to 50% of the total resistance of the aircraft. How to reduce the friction resistance is a problem that aircraft designers have been thinking about. Therefore, it is of great significance to measure the friction resistance.

[0003] Friction resistance is the integral of the overall surface friction stress. The commonly said friction resistance measurement refers to the measurement of the friction stress of the specified position or area of the model surface. The friction stress measurement methods are generally divided into direct measurement and indirect measurement. Direct measurement methods include drag balance, oil film method and shear sensitive liquid crystal method, etc. There are many indirect measurement techniques, mainly including Preston tube, Stanton tube, bottom layer baffle method, hot wire, hot film, velocity type method, etc. Among them, the oil film method and the shear sensitive liquid crystal method can be globally measured, and other methods can only be locally measured.

[0004] The oil film method measures the model surface friction stress based on the oil film equation. The equation was originally proposed by Squire and describes the movement law of the oil film under the action of shear stress. By measuring the change of the oil film thickness with time and space, the friction stress can be calculated. According to its measurement principle, it is crucial to accurately measure the oil film thickness. At present, there are three methods for measuring the oil film thickness: interference method, reflection method and fluorescence method. These methods have strict requirements for the model surface state and the optical measurement channel of the wind tunnel, and the solving method is relatively complex.

[0005] The shear sensitive liquid crystal method utilizes the principle that the liquid crystal film has unique optical properties under the action of surface friction stress. When the liquid crystal film is subjected to shear force, the birefringence characteristics or circular dichroism of the liquid crystal change, thereby causing the polarization light passing through it to rotate and selectively reflecting the reflected light from the lower layer of the liquid crystal. This selective reflection causes the color of the liquid crystal film to change. Through calibration, the corresponding relationship between the color change of the liquid crystal film and the friction stress is obtained. In the experiment, the color image of the liquid crystal film is recorded, and based on the calibration relationship, the corresponding friction stress can be calculated. The shear sensitive liquid crystal method needs to provide enough observation angles, and the equipment arrangement and solving method are relatively complex.

[0006] Generally speaking, most of the current friction stress measurement methods are local measurement methods, which are relatively mature in technology and relatively simple in equipment installation, while the global measurement method requires higher hardware and more complex calculation method. SUMMARY

[0007] The purpose of the present application is to measure the global friction stress by heating the model surface with a coating heating method, obtaining the temperature distribution of the model surface with a high-precision infrared thermal imager after reaching thermal equilibrium, and obtaining the local friction stress at the specified position with the mature local friction stress measurement technology, and calculating the friction stress along the streamline based on the friction stress and the temperature distribution along the streamline.

[0008] In order to achieve the above purpose, the present application adopts the following technical solutions:

[0009] Step one: Obtain the streamline diagram of the model surface. The surface streamline can be obtained by numerical simulation, experimental measurement or theoretical estimation.

[0010] Step two: Prepare the heating coating. Spray the heating coating on the model surface, which is divided into three layers: upper, middle and lower. The upper and lower layers are insulating layers, which require good insulating and heat insulating properties, such as polyurethane insulating paint. The middle layer is a heating layer, which requires strong adhesion, excellent heating performance, good elasticity, heat aging resistance and other advantages, such as carbon nanotube conductive water-based paint. The thickness of the entire heating coating is only a few tens of microns, and the coating surface has good smoothness, and the heating uniformity of the coating is good.

[0011] Step three: Install the local friction stress measurement equipment. Install the local friction stress measurement equipment at the starting position of the measured friction stress area along the flow direction, which requires that the measurement method has little effect on the downstream flow, such as MEMS drag balance, hot film, etc.

[0012] Step four: Obtain the infrared thermal image of the model surface. In the wind tunnel experiment, the heating coating is powered to heat the model surface, and the heating power is kept constant. After the model surface reaches thermal equilibrium, a high-precision infrared measurement system is used to shoot the infrared thermal image of the model surface. The temperature formula calibrated in advance is used to calculate the temperature distribution of the model surface.

[0013] Step five: Obtain the local friction stress result. Collect the output signal of the local friction stress measurement equipment at the same time as step four, and calculate the friction stress at the specified position based on the pre-calibrated friction stress measurement formula.

[0014] Step six: Obtain the friction stress distribution along the streamline. Calculate the friction stress of each point along the streamline step by step by formula (1). Wherein, is the friction stress, unit Pa, T is the temperature, unit K, is a constant small quantity, i is the current point, and i+1 is the next point along the streamline. The point at i=0 is measured as the local frictional stress measurement position, and each term on the right side of equation (1) is a known condition, so the frictional stress can be calculated . By analogy, the frictional stress on the entire streamline can be calculated.

[0015] (1)

[0016] In summary, due to the adoption of the technical solutions described above, the present application has the following beneficial effects:

[0017] The global frictional stress is measured by using the mature local frictional stress measurement method and high-precision infrared thermal imaging technology, the measuring equipment is relatively simple, the model surface is heated by a heating coating at a constant power, the frictional stress size is related to the local temperature, the calculation method is relatively simple, the resolution of the frictional stress is high, and is comparable to the resolution of the infrared image, and other flow field information can be obtained at the same time, such as determining the transition position through the infrared thermal image and determining the separation position through the frictional stress size. BRIEF DESCRIPTION OF DRAWINGS

[0018] The present application will be described by way of example and with reference to the accompanying drawings, in which:

[0019] Figure 1 is a schematic diagram of heating of two adjacent points;

[0020] Figure 2 is a schematic diagram of single-point heat transfer;

[0021] Figure 3 is a schematic diagram of a wing surface streamline;

[0022] Figure 4 is a schematic diagram of a heating coating;

[0023] Figure 5 is a schematic diagram of installation of a local frictional stress measurement device;

[0024] Figure 6 is a temperature distribution on the streamline;

[0025] Figure 7 is a frictional stress distribution on the streamline. DETAILED DESCRIPTION

[0026] All features disclosed in this specification, or all steps of any methods or processes disclosed, may be combined in any combination, except combinations where at least some of the features and / or steps are mutually exclusive.

[0027] Any feature in the foregoing specification that can be added to the application, whether preferred or not, can be deemed insubstantial form. That is, any feature in the foregoing specification that can be added to the application, whether preferred or not, can be deemed insubstantial form.

[0028] As Figure 1 shown, two adjacent points on the streamline heat the air flow in the boundary layer. Wherein, v represents the velocity, T represents the temperature, and subscript a represents the gas. Since it is constant power heating and uniform heating, the heat energy transferred into the two dashed boxes is the same after thermal equilibrium. Therefore, the heat energy obtained by the gas passing through the two dashed boxes per unit time is also the same, that is

[0029] ,

[0030] Thus

[0031] (2)

[0032] The single-point heat transfer condition is shown in Figure 2 For the dashed box, the heat energy transferred in and out is in balance, so

[0033]

[0034] Arranging the above formula, we have

[0035]

[0036] Integrating the above formula, we have

[0037] (3)

[0038] Substituting formula (2) into formula (3), we have

[0039] (4)

[0040] From the definition formula of friction stress

[0041]

[0042] We can approximately get Substitute it into formula (4) and arrange it to have

[0043]

[0044]

[0045] Assuming that the friction stress τi at point i is known, and the temperatures at points i and i+1 are also known, the friction stress at point i+1 can be calculated by the above formula .

[0046] Take a certain two-dimensional flat wing as an example for illustration:

[0047] As shown in Figure 3 , the flow line diagram of the wing surface is obtained through numerical simulation; as shown in Figure 4 , the heating coating is sprayed on the wing surface, the lower insulation layer is first covered on the surface of the model by spraying, and after solidification, the heating layer is sprayed on the surface of the lower insulation layer, and is solidified by drying or other methods, and finally the upper insulation layer is sprayed and solidified. After the coating is completed, the surface is polished to ensure that the surface roughness is not more than 1.2 μm to meet the requirements of the model surface roughness of the wind tunnel experiment. When spraying the heating coating, it should be ensured that the thickness of each layer is uniform, and the thickness of the entire coating is about several tens of microns. The insulation layer should have good insulation and heat insulation performance, and the heating layer should have good heating and heat aging resistance.

[0048] As shown in Figure 5 , the local friction stress measuring device is installed at the specified position, and the MEMS friction balance is installed at the 20% chord length of the wing span in the middle position of the embodiment to measure the local friction stress. The MEMS balance needs to be flush with the surface of the model.

[0049] The temperature distribution on the flow line passing through the MEMS friction balance is obtained. Start the wind tunnel experiment, heat the model surface by the heating coating at a specified power, and monitor the temperature change of the model surface by the high-precision infrared measurement system; when the model surface reaches thermal equilibrium, the infrared thermal image of the model surface is taken; based on the calibrated infrared temperature formula in advance, the temperature distribution of the model surface is calculated, and the temperature value on the flow line passing through the MEMS friction balance is extracted, as shown in Figure 6 .

[0050] At the same time of taking the infrared thermal image of the model surface, the output signal of the MEMS friction balance is collected, and based on the calibration formula of the MEMS friction balance, the friction stress at the 20% chord length of the wing span in the middle position is calculated as 50.3 Pa.

[0051] The friction stress value along the flow line is calculated as shown in Figure 5 , based on the friction stress at the 20% chord length (50.3 Pa) and the temperature values at the point and the next point, the friction stress at the next point is obtained according to formula (1), and the calculation is carried out point by point, and finally the friction stress distribution in the range of 0.2~1.0 can be obtained. x c =0.2~1.0.

[0052] ​The application is not restricted to the foregoing specific embodiments. The application extends to any novel one, or any novel combination, of the features disclosed in this specification, and to any novel method or process disclosed in this specification or any novel combination thereof.

Claims

1. A method for global measurement of surface frictional stress of a wind tunnel model, characterized in that... Includes the following steps: Step 1: Obtain the streamline diagram of the model surface through numerical simulation, experimental measurement, or theoretical estimation; Step 2: Spray three heating layers (top, middle, and bottom) onto the model surface, where the top and bottom layers are insulating layers and the middle layer is the heating layer; Step 3: Install a local friction stress measuring device at the starting position along the flow direction in the area where the friction stress to be measured is to be measured; Step 4: In the wind tunnel test, electricity is applied to the heating coating to heat the model surface. After the model surface reaches thermal equilibrium, an infrared thermal image of the model surface is taken. Step 5: Simultaneously with Step 4, acquire the output signal of the local friction stress measuring device, and calculate the friction stress at the specified location based on the pre-calibrated friction stress measurement formula; Step Six: Calculate the frictional stress at each point along the streamline step by step using the following formula: ,in: Here, T represents frictional stress, and T represents temperature. It is a small constant. For the current point, This is the next point along the streamline.

2. The method for global measurement of surface frictional stress of a wind tunnel model according to claim 1, characterized in that... The surface roughness of the coating on the model surface does not exceed 1.2 μm.

3. The method for global measurement of surface frictional stress of a wind tunnel model according to claim 2, characterized in that... The thickness of each layer of the coating on the model surface is uniform.

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