拖网模型试验小尺度雷诺相似准则的理论探讨

A STUDY ON SMALL--SCALED REYNOLDS SIMILARITY CRITERION FOR MODEL TEST OF THE BODY OF TRAWL-NET

本文从理论上探讨拖网模型试验的相似准则。首先分析现有相似准则的理论前提中存在的二个问题,然后提出一种以拖网网体的小尺度雷诺相似为基础的准则。这一准则与现有准则有二个主要的不同点:(1)现有准则假定网具阻力与速度平方成正比,亦即阻力系数是常数K与雷诺数无关,试验中不考虑粘性力相似条件。本准则认为网具阻力系数C_R不是常数而是雷诺数的函数,亦即C_R=C_R(R_e)。在模型试验中考虑粘性力相似条件。(2)现有准则按重力相似,亦即相当于网体小尺度傅汝德数相似条件推导实网与模型网之间的速度关系。本准则认为支配网具阻力的主要因素是粘性力。考虑到网体的滤流特点,以网体的小尺度雷诺相似为条件推导实网与模型网之间的速度关系。同时考虑到二者的大尺度雷诺数不同所产生的尺度效应,引进了相应的修正系数。在讨论了相似准则的理论前提后,本文阐述了新准则的要点。

This paper is a study, on small-scale Reynolds similarity criterion for model testof the trawl--net. After analysing the theoretical basis for current similaritycriteria, it gives a new similarity criterion based on small-scaled Reynolds similarity. 1. The main differences between the two kinds of criteria are as follows: (1) The current criteria do not consider the viscous force similarity. They supposethat the coefficient of net resistance is a constant, and the net resistance is proportionalto the square of velocity. In this criterion. we consider the coefficient of net resistanec (C_R) is not a constant but a function of Reynolds number, and then the viscous forcesimilarity criterion should be considered. (2) In the current criteria, the relation between the velocity of model net and fullscale net is introduced according to the gravity force similarity. But, according to thefilter-flow speciality of net, we think viscous force is the most important one for netresistance. We take the small-scaled Reynolds numbers similarity as a basis forintroducing the relation between the velocity of model net and full scale net. Simul-taneously, a revision coefficient (C_1) is introduced to consider the difference betweentwo large-scaled Reynolds numbers. 2. In this paper, the main points of the criterion are pointed out. The relationsbetween model net and full scale net are as follows: Large scale rstio: l_1/l_2 = b_1/b_2 =H_1/H_2=... = λ Small scale ratio: d_1/d_2 =a_1/a_2 = S_2'^(1/2)/S_2'^(1/2)=...=λ' Large-scaled Reynolds numbers: Re_1 = V_1l_1/v_1, Re_2 = V_2l_2/v_2 Small-scaled Reynolds numbers: Re_1' = V_1d_1/v_2, Re_2'= V_2d_2/v_2 The two small--scaled Reynolds numbers are equal: Re_1' = Re_2' The two large--scaled Regnolds numbers are not equal: Re_1≠Re_2 The relation between two velocities: V_2 = V_1λ' The formu1as of net resistance: R_1 = 1/2ρ_1V_1~2Ω_1C_(R_1), R_2 = 1/2ρ_2V_2~2Ω_2C_(R_2) The relation between two resistance coefficients: C_(R_1)=C_1C_R The coefficient C_2 is a revision coefficient. It considers the difference between twolarge-scaled Reynolds numbers. If Re_2'≥Re_(kp), C_1→1, Re_(kp) is a critical Reynolds number. In the above, '1' is for full scale net; '2' is for model net.