GB/T 47707-2026Test method for distributed friction force measurement in air layer drag reduction models (English PDF)

2 Normative references

This document has no normative references.

The symbols given in Table 1 are applicable to this document.

5 Experimental Equipment and Instruments

5.1 Variable Pressure and Variable Speed Circulating Water Tank Circulating water tanks used for measuring the gas layer distribution resistance at a model scale of 10m or more should meet the following requirements.

a) The test section is over 10m long;

b) Width and height both exceeding 1m;

c) The maximum water velocity must not exceed 10 m/s;

d) Maximum pressure adjustment exceeds 300 kPa.

7 Installation Requirements for Sensors and Test/Observation Equipment

7.1 Installation Requirements for Local Resistance Measurement Sensors The local resistance measurement sensors are aligned with the model's centerline and are positioned in a straight line within the cavity, parallel to the model's longitudinal axis. Multiple resistance sensors are connected at one end to the unit force plate and at the other end to the fixed base inside the model. The unit force plate is also connected to the flat plate model. The gap is approximately 1mm. A sealed cavity and partition are installed above the force measuring plate and resistance sensor to ensure that gas enters through the gap without leaking out. The gas accumulates and reaches equilibrium before stabilizing. Typical installation examples in the water tank are shown in Figures A.3 and A.4.

7.2 Installation Requirements for Pulsating Pressure Sensors The pulsating pressure sensor is rationally selected based on the area where gas layers may form, and embedded on the plate where the gas cavitation is located, with its surface flush with the plate where the gas cavitation is located. The plate surface is flush and the edges are smooth without any bumps. A pulsating pressure sensor can also be embedded in the unit force plate, with the pressure-sensing surface of the pulsating pressure sensor aligned with... The lower surface of the force measuring plate is flush with the surface.

8 Pre-test measurement requirements

8.1 Requirements for Test Equipment and Instruments Before the test, the test equipment and instruments should meet the following requirements.

a) The test water is clean, transparent, and has a relative air content of less than 0.85;

b) The hull/flat plate and gas layer generation device models must be uniquely identified;

c) All instruments and meters used in the test are within their metrological validity period;

d) Before measurement, the instruments and meters should be turned on and warmed up for at least 15 minutes.

8.2 Determination of Experimental Parameters The determination of test parameters should meet the following requirements.

a) Test pressure. Ensure that during the jetting process, the ambient pressure in the test section is less than the maximum range of the pulsating pressure sensor or the pressure overload. ability;

b) Test water velocity. meets the supercritical Reynolds number, the specified test flow velocity, or the actual operating speed of the ship.

9 Test Procedure

9.1 Model Assembly The specific requirements for assembling the test model are as follows:

a) The hull/flat plate model is hoisted into the test section, ensuring that the bow and stern centerlines of the model are parallel to the centerline of the test section, i.e., the bow/stern of the model. The difference in distance between the centerline and one side wall of the test section is less than 2mm, and the difference in distance between the bow/stern centerlines and the test section is also less than 2mm. The bottom height difference is less than 2mm;

b) When installing the force plate and pulsating pressure sensor of the local resistance unit, ensure that the pressure-sensing surface of the force plate and pressure sensor is in contact with the cavitation. The inner model surface is flush, and there are no burrs or rough spots around the test area;

c) Inspect the cavitation area and the area between the baffle and the model for any holes, cracks, gaps, etc. indicating gas leakage;

d) Connect the gas source, gas flow meter, pipelines and piping systems, pressure stabilizing chamber, nozzles, etc., and perform gas injection and inspection to ensure that the entire gas pipeline system, except for the nozzles, is functioning properly. There was no air leakage outside the nozzles, and the gas velocity ejected from each nozzle was uniform and stable. This can be verified using an anemometer or by filling the nozzles with water. Check the movement status of the ejected gas.

9.2 Integrated Testing of the Test System To reduce the impact of pressure changes during the jetting process on the testing instruments and to ensure proper gas selection under different gas layer conditions during subsequent experiments. Traffic settings should be configured as follows, and the test system should be integrated and debugged according to the following steps.

a) Before the experiment, at a certain flow rate, jet gas was injected into the cavitation, and the uniformity and stability of the ejected gas were observed. Simultaneously, the flow rate of the gas from the cavitation head was also observed. After the gas is ejected, ensure that there is no gas floating or leaking within the entire cavitation area;

b) Test several flow rates and perform gas injection at multiple flow rates to check the safety and reliability of the entire model, while also testing the injection... During the gasification process, the increase in environmental pressure in the water tank test section;

c) Under normal operating conditions of all equipment in water, divide the range of environmental pressure increase from step

b) into 3 to 4 intervals. When v=0, test the sensitivity of each local resistance sensor to pressure changes, and calibrate the force sensor by specifying a certain pressure. The stability of the output when the pressure rises and falls back to the specified pressure is measured, and this is obtained through repeated measurements of the environmental conditions. The effect curve of pressure on the local resistance sensor;

d) Determine the approximate gas flow rate required for the formation of a gas layer (or local gas layer) in the cavitation region under several different flow rate conditions;

e) Check whether the pulsating pressure sensor works properly and has good performance under different flow rates and conditions of full humidity, bubbles, and gas layers. Resolution;

f) Check whether the scale markings on both sides of the baffle are reasonable and whether they can be clearly distinguished and read under typical working conditions and different lengths of the gas layer.

9.3 Test Measurement The test measurements should be performed according to the following steps.

10 Experimental Data Processing and Presentation

10.1 Data Processing and Representation of Local Resistance Measurement Based on the voltage signals measured during the experiment, and using the calibration coefficients and pressure correction curves of the local resistance sensors at various locations, the following calculations were performed. Calculate the drag of each unit force sensor at different speeds with and without jet propulsion, and calculate the corresponding drag coefficient. Use the Haisang formula to calculate the drag of each unit. The drag coefficient of the force sensor at the corresponding location under fully wet conditions. Compare the consistency between the measured and calculated results under fully wet conditions. Within reasonable limits, and to confirm the validity of other measurement data during the jetting process.

10.2 Data Processing and Representation of Pulsating Pressure Measurement The surface pulsating pressure under different gas layer conditions is measured by a piezoelectric sensor, and the pulsating pressure is obtained by energy integration within a certain frequency range. The overall level, i.e., the pulsating pressure results under different operating conditions, is given in the form of fm~SPL(fm)~SPL. Where SPL is calculated according to formula (8). calculate.

10.3 Data Processing and Expression of Gas Layer Morphology Measurement Results The gas layer results are presented primarily through a combination of hand-drawn diagrams and local video imagery, depicting the gas layer under different conditions on the flat bottom. The shape of the surface. First, draw the projected outline of the experimental model, including the head, the air cavitation plane area, and the tail, as well as the flat bottom where the air cavities are located. The surface can be represented by a single layer of oblique lines to indicate a smooth air layer coverage area, and by a double layer of oblique lines to indicate a rough, fish-scale-like air layer coverage area. The dashed lines represent gas escape barriers, and the curves represent gas layer thickness at different locations. Furthermore, for specific gas layer conditions, different flat plates... Images of the gas layer (surface morphology, thickness, and wave shape at the gas-water interface, etc.) are captured at the location to aid in explanation and to compare with the gas layer morphology. The drawings corroborate each other.

11 Gas Layer State Identification Method

11.1 General Provisions In gas layer drag reduction measurement experiments, because the physical parameters measured within the gas layer differ during its formation, the methods for identifying the gas layer state also vary. The following section presents three typical methods for gas layer state identification. Other methods for gas layer identification include porosity meters and conductivity meters. All methods can refer to the method for measuring and identifying the pulsating pressure on the inner wall of the gas layer for measurement and analysis.

11.2 Drag Reduction Rate Changes and Gas Layer Identification Methods The variation of lower surface resistance measured by a local unit force plate on a flat-bottomed or extra-large flat plate of a ship under fully wet flow conditions and different jet flow conditions. Based on the calculation results of formula (4), the change in drag reduction rate can determine whether a gas layer has formed. For the local drag measurement gas layer identification method, when the drag reduction rate is reduced... When the resistivity is less than 20%, it is in the bubble (including microbubble) drag reduction stage (BDR); when the drag reduction rate is between 30% and 60%... Within the specified range, it represents the transition state between the bubble and the gas layer; when the drag reduction rate exceeds 80% (inclusive), it represents the drag-reduced state of the gas layer. (Air layer degeneration, ALDR). See Figures A.5 to A.8.

11.3 Methods for identifying water vapor patterns and gas layers Water vapor speciation and gas layer identification methods should meet the following requirements.

a) Using optical measurement methods, images of bubbles with different densities are displayed on the bottom surface of the flat plate during the bubble drag reduction stage;

b) During the transition phase, after the water and air are fully mixed, a "glassy" lubricating layer is formed;

c) During the gas layer stage, there are a small number of droplets on the lower surface of the observation window. Under the action of airflow, the droplets can move slowly. While transparent baffles and graduated lines on both sides of the cavitation can distinguish between gas layers, it is difficult to differentiate between bubble morphology, transition zone morphology, and gas layer morphology. The process involves three stages. See Figures A.9 to A.15 for relevant diagrams and photographs.

11.4 Gas Layer Inner Wall Pulsating Pressure and Gas Layer Identification Method Spectral analysis of experimental data obtained using a piezoelectric pulsating pressure sensor can distinguish between gas layers and non-gas layers. During gas layer formation, the pulsation... The dynamic pressure spectrum curve has a distinct convex envelope, and the total pulsating pressure level is higher than that in the fully wet state within the.200Hz~8000Hz frequency band. The total pulsating pressure level is more than 10 dB higher. See Figures A.16 and A.17. Other methods for gas layer identification include porosity meters, conductivity meters, and other measurement and identification techniques. These can all be referenced in the measurement and identification of pulsating pressure on the inner wall of the gas layer. Other methods are used for measurement and analysis.

12 Test Report The test report should include the following.

a) Test number and name;

b) Testing instruments and equipment;

d) Experimental objective;

e) Testing basis;

f) Specimen test type;

g) Test installation and test conditions;