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为什么定义反向空气动力阻力?

2014 - 09 - 08 - t02:58:56z

当我想到风的阻力,我总是认为摩擦系数,在美元\ mathrm{女士^{1}}$,因为这总方式提出了在高中和本科物理。 但在陆地表面造型,空气动力阻力(基本上,多少的粗糙度表面空气流动放缓下来),反向定义。例如,< a href = " http://www.iac.ethz.ch/edu/courses/master/modules/land-climate_interactions/HS2013/LCI13_lecture3.pdf " rel =“nofollow”>这个演讲< / >州18页: < blockquote > < ul > <李> r_a f = $美元(美元风速、大气稳定,表面粗糙度$)< /李> <李> r_a美元美元随风速增大而减小李< / > <李> r_a增加而增加美元稳定< /李> <李> r_a随美元增加表面粗糙度李< / > < / ul > < /引用> ,然后给出了很多例子在美元\ mathrm {sm ^ {1}} $: < pre > <代码>表面型ra (s.m ^ 1)海洋200草70作物20 - 50森林5 - 10 < /代码> < / pre > 对我来说,似乎真的反直觉(“抵抗高草比森林”?)。定义的一些好处是抵抗这种方式,或者这只是一些烦人的历史错误?

回答由Deditos反向空气动力阻力的定义是为什么呢?

2014 - 09 - 08 - t21:02:44z

部分历史,部分观点,但它不是一个错误。 摩擦系数的影响强调表面边界层的特性,即。,更大的表面摩擦放缓近地表风。空气动力阻力强调在surface-atmosphere交易所边界层的影响,即。,更大的混合收益率较低的电阻蒸发。注意,蒸发不是平流的抵制。你可以扭转混乱,认为摩擦系数是一个反直觉的术语:如果有更多的摩擦,为什么蒸发增加?这将是使同一类别错误。 历史上(这个)电路由笔者介绍了隐喻和斯科菲尔德(1951)在早期修改笔者蒸散方程占植被如何限制了蒸腾速率。原画家方程往往是引用使用气动电导(或摩擦速度或摩擦系数),即量使用$ m s ^{1} $单位,你习惯。micrometeorologists试图理解这是一个自然选择的湍流边界层的传质性能,因为它可以从边界层计算量测量。 一旦你开始引入其他表面对蒸发的影响不是适当的延长的类比湍流边界层的摩擦速度下植物气孔,根和土壤。 So we use the electrical metaphor instead, in which case it becomes slightly simpler to describe the system using resistances rather than conductances. In the first instance it was just a case of calculating the total system resistance by adding a stomatal resistance to the aerodynamic resistance:

$E = \frac{\rho}{r_s+r_a}\left(q_{sat} - q_a\right) = \frac{\rho}{r_t}\left(q_{sat} - q_a\right)$

You can express this as conductances, but it's not as neat:

$E = \frac{g_a g_s \rho}{g_a+g_s}\left(q_{sat} - q_a\right)$

You can see in the later slides of the presentation you linked to that these resistance networks have got progressively more complex, so nowadays this neatness point might be moot.

Penman and Schofield (1951) Some physical aspects of assimilation and transpiration, Symp. Soc. Exper. Biol., 5, 115-129.

回答由milancurcic反向空气动力阻力的定义是为什么呢?

2014 - 09 - 09 - t15:18:28z

这是一个好问题,答案是,空气动力阻力是<强> < /强>定义反向。而是,定义在一个上下文通常是误解。 在你的问题,你的国家,空气动力阻力基本上是多少的粗糙度表面空气流动放缓下来。这种说法是不正确的,它似乎源于上下文的误解。 De Groot的专著(1963)还表明,分子转移过程的一般形式类似于电路所示@Deditos”答: $ $ =通量\ dfrac{力}{阻力}$ $ 这也是对海气界面和其他通用油接口。通量是一个量的转移(动量,焓、质量等)通过接口和相关的边界层(说,空气、水、树冠、土壤,等等)。电路类比,迫使电位梯度的特点,耐逆的导电性。重要的一点,就是抵抗<强>不是< /强>界面到空气动力流阻力——我们直观地想象摩擦或压力。事实上,接口的< >的强烈反对,迫使< /强>。的势头,这意味着同等的迫使,高电阻收益率低通量。 Thus, lower resistance translates to rougher surface. This is why forest has lower resistance values than grass or open ocean.

Example: Given equal forcing, rougher surface results in higher stress compared to smoother surface. It can be said that the rougher surface is "more permitting", or "less resistant" of momentum flux.

In the electric circuit analogy, Flux, Force and Resistance are symbolic, conceptual entities. Force is not necessarily in $\rm N$, and may be a temperature or humidity gradient like it is given on slide 16 in the presentation you linked. Resistance may thus take different formulations.

Note that nowadays, in both modeling and theory, we often use exchange coefficients to characterize momentum $(C_{D})$ and enthalpy $(C_H$, $C_E)$ fluxes through the interface, which act as conductivity and not resistance. For example, in case of momentum:

$$ \boldsymbol{\tau} = \rho C_{D}|\mathbf{U}|\mathbf{U} $$

where $\boldsymbol{\tau}$ $(\mathrm{N/m^{2}})$ is vertical flux of horizontal momentum (wind stress), $\rho$ $(\mathrm{kg/m^{3}})$ is air density and $\mathbf{U}$ $(\mathrm{m/s})$ is wind vector at some reference height above the surface. $C_{D}$ (non-dimensional) has different values depending on the surface properties.

In that particular presentation that you linked in your question, it is not clear to me why resistance $r_a$ has units of $\mathrm{s\ m^{-1}}$. For sensible and latent heat flux $\mathrm{ (W / m^2) }$ formulations on slide 16, the units don't quite work out, but it is possible that the equations shown were more illustrative than exact. Because bulk flux formulae are most often based on theoretical, empirical and dimensional grounds, $r_{a}$ can be defined in various dimensions (units) depending on the bulk flux formulation.

Reference:

De Groot, S. R. Thermodynamics of Irreversible Processes. North Holland Publishing Co., 1963.